Ferroelectrics Dielectric Spectroscopy of Polymer

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Dielectric Spectroscopy of Polymer Based PDMS Nanocomposites with ZnO Nanoparticles a

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J. Belovickis , J. Macutkevic , Š. Svirskas , V. Samulionis , J. Banys , b

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O. Shenderova & V. Borjanovic a

Faculty of Physics, Vilnius University, Sauletekio 9/3, LT-10222, Vilnius, Lithuania b

International Technology Center, Raleigh, NC, 27715, USA Published online: 09 Jun 2015.

Click for updates To cite this article: J. Belovickis, J. Macutkevic, Š. Svirskas, V. Samulionis, J. Banys, O. Shenderova & V. Borjanovic (2015) Dielectric Spectroscopy of Polymer Based PDMS Nanocomposites with ZnO Nanoparticles, Ferroelectrics, 479:1, 82-89, DOI: 10.1080/00150193.2015.1012016 To link to this article: http://dx.doi.org/10.1080/00150193.2015.1012016

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Ferroelectrics, 479: 82–89, 2015 Copyright Ó Taylor & Francis Group, LLC ISSN: 0015-0193 print / 1563-5112 online DOI: 10.1080/00150193.2015.1012016

Dielectric Spectroscopy of Polymer Based PDMS Nanocomposites with ZnO Nanoparticles SVIRSKAS,1 J. BELOVICKIS,1,* J. MACUTKEVIC,1 S. V. SAMULIONIS,1 J. BANYS,1 O. SHENDEROVA,2 AND V. BORJANOVIC2


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Faculty of Physics, Vilnius University, Sauletekio 9/3, LT-10222 Vilnius, Lithuania 2 International Technology Center, Raleigh, NC 27715, USA Dielectric spectroscopy results of organic polymer Polydimethylsiloxane (PDMS) with different ZnO nanoparticle concentrations are presented in wide temperature range. Dielectric loss was measured as a function of electromagnetic field frequency at various temperatures. Our measurements have shown that the value and position of dielectric loss peak on a temperature scale depends on the concentration of ZnO nanoparticles within PDMS. The temperature behaviour of dielectric losses was compared with temperature dependencies of ultrasonic attenuation in the samples. It is shown that the maximum of dielectric losses in PDMS nanocomposite is described according to the Vogel-Fulcher law and the glass transition increases with ZnO concentration. Keywords Polydimethylsiloxane; zinc oxide; polymer; nanocomposite; glass transition; dielectric relaxation; Vogel-Fulcher relation

1. Introduction Polymer silicones were commercialized in the United States in the early 1940s and have experienced strong growth ever since. Small amounts of neat Polydimethylsiloxane (PDMS) were used with other commercial silicones as additives in solvent-based coatings to prevent surface defects such as scattering [1]. Nowadays PDMS is one of the most used polymers in chemical and biological applications due to its optical transparency, mechanical compliance, chemical stability, bio-compatibility, and ease of fabrication [2]. PDMS is a semicrystalline polymer, therefore a glass transition is observed in this polymer. At the glass transition temperature the polymer goes from a rubber-like to a hard glass-like state [3]. The value of glass transition temperature for PDMS polymer (TgD 148 K) is one of the lowest observed in polymers [4]. At room temperature the pure PDMS is a good electrical isolator [5].To become electrically conducting PDMS should be doped, for example, with conducting nanofillers [6]. Among other nanofillers (like carbon nanotubes, carbon black and others) which are often used for polymer composite preparation, ZnO nanoparticles and nanorods are very interesting due their unique semiconducting, photoconducting and piezoelectric properties Received July 8, 2014; in ﬁnal form August 11, 2014. *Corresponding author. E-mail: [email protected]

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Dielectric Spectroscopy of ZnO/PDMS Composites

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[7, 8]. Although the electrical and mechanical properties of ZnO nanomaterials have been extensively investigated, the investigations of dielectric properties of PDMS composites filled with ZnO nanoparticles are rather rare in the literature [5, 9, 10]. PDMS and ZnO nanocomposites were the choice of our study because of its low cost and wide availability. The goal of the work is to study the influence of ZnO nanoparticles on the nanocomposite dielectric properties in a wide temperature range.


2. Experimental Procedure Fractions of ZnO with aggregate sizes 30 nm were used as filler similarly as in the literature [11]. Dow-Corning supplied us with the polydimethylsiloxane, Sylgard, as a two part material. Forming procedure of ZnO/PDMS composites was as follows: the ZnO nanoparticles were dispersed in an isopropanol solvent and sonicated to break up large agglomerates, after that the suspension was mixed with uncured PDMS. The isopropanol solvent was removed by vacuum. The mixture of PDMS and nanoparticles was cured at 60 C for several hours and at 40 C overnight. Such procedure allowed us to obtain nanocomposites with good nanoparticle dispersion. Investigation of dielectric relaxation in PDMS loaded with ZnO nanofillers (30 nm) of different concentrations (pure PDMS, 1 wt% and 5 wt% of ZnO inclusions in PDMS) was performed in wide frequency 20 Hz–1 MHz and temperature 108–300 K ranges. The thickness of the pure PDMS, 1 wt% and 5 wt% samples was 3.5 mm, 3.13 mm and 2.32 mm, respectively. The complex dielectric permittivity of ZnO/PDMS nanocomposite was measured as a function of frequency and temperature using a bridge setup with LCR meter HP4284A. A multimeter Keithley Integra 2700 was used for precise temperature determination. Silver paste was used for electrical contacts. Measurements have been performed on heating and cooling at a rate of about 1 K/min. Each measurement started from cooling, and after reaching 100 K, sample was heated up to room temperature. Ultrasonic measurements were performed by automatic pulse-echo method similarly as the polyurethane composites with inorganic particles [12]. The sample was carried between two ultrasonic waveguides lubricated with bulk silicone oil to glue the sample and waveguides. Acoustic wave transducers were used for transmitting and receiving an acoustic wave, respectively. More details about ultrasonic measurements technique are in [13].

3. Results and Discussion Temperature dependence of real (e0 ) and imaginary (e00 ) part of complex dielectric permittivity (e*D e0 -ie00 ) for pure PDMS and composites with 1 wt% and 5 wt% of ZnO inclusions at frequency 1 MHz is shown in Figure 1. The behaviour of complex dielectric permittivity as the primary (a) relaxation is observed at temperatures close to 175 K. The dielectric permittivity of composites is almost temperature independent above 175 K temperature for composites and slightly increases on cooling for pure PDMS (Figure 1a). The differences in temperature behaviour of composites and pure polymer matrix can be explained by different area sizes of composites and pure PDMS, the lower area of pure PDMS sample causes a higher pressure on sample and more rapid sample shrinkage on cooling. The sample shrinkage on cooling causes an increase of sample capacitance and effectively the increase of dielectric permittivity.

J. Belovickis et al.


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Figure 1. Temperature dependence of real (a) and imaginary (b) part of complex dielectric permittivity for pure PDMS and for composites with 1 wt% and 5 wt% ZnO inclusions on cooling at 1 MHz frequency.

It is clearly observed that the peak of dielectric loss maximum (Tmax) shifts to higher temperature values with increasing the concentration of Zinc Oxide particles in the composite (Figure 1b). Dielectric dispersion and the peak of dielectric losses have been observed near glass transition temperature. Figure 2 exhibits the dependence of the real (a) and imaginary (b) part of dielectric permittivity for 5 wt% ZnO in PDMS on the electromagnetic field frequency close to the glass transition temperature Tg. The curves of losses show a single relaxation peak in the temperature range 1 kHz–1 MHz. The dielectric dispersion is caused by the a relaxation for PDMS, which nature is related with the local segmental dynamics [14, 15].



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Figure 2. Variation for real (a) and imaginary (b) part of dielectric permittivity for 5 wt% ZnO in PDMS with frequency at different temperatures close to Tg (160–180 K).

It can be observed from Figure 3a that temperature dependencies of imaginary part of complex dielectric permittivity in heating and cooling runs does not coincide (Figure 3a). Such temperature hysteresis was also observed in ultrasonic attenuation dependence on temperature (Figure 3b). The temperature dependence attributed to the glass transition followed by the a relaxation in the cooling cycle and the complex melting on heating in the range 220–230 K, was in good agreement with previous PDMS polymers investigations [16]. We attribute these different dielectric loss dependencies of ZnO/PDMS nanocomposites to the first order thermal transition (crystallization or melting), which is observed on heating [17]. We suggest that the nanocomposite does not crystallize during cooling at low rates as 1 K/min. Upon heating due to chain rearrangements and the decrease of melt viscosity PDMS crystallizes from the amorphous phase [17]. As the amount of nanoparticles affects the crystallite size, degree of crystallinity, the melting is also enhanced by the presence of Zinc Oxide nanoparticles resulting in the dependence of the hysteresis shape on ZnO concentration in the PDMS as it was previously studied [17].


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Figure 3. Temperature dependencies of imaginary part of complex dielectric permittivity (a) and ultrasonic wave attenuation (b) for composites with 5 wt% of ZnO inclusions. Solid lines: on cooling, dashed lines: on heating.

Figure 4 illustrates the relationship between the losses for composites with 5 wt% of ZnO inclusions and the temperature at different frequencies (11.4 kHz–1 MHz). The peak of the temperature-dependent losses increases and shifts to higher temperatures with the frequency of the measurement. The behaviour was accurately described by the Vogel–Fulcher relationship [18, 19]: ¡ Ea

f D f0 ek.Tmax ¡ Tref / ;

(1)

where k is the Boltzmann constant, f0 is the frequency approached with Tm!1, Ea is the activation energy, and Tref is the temperature of glass transition in the polymer.



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Figure 4. Temperature dependence of the losses at different frequencies (11.4 kHz – 1 MHz).

Figure 5 shows experimental and fitted values of frequency versus the dielectric loss maxima temperature according to the Vogel-Fulcher equation (Eq. 1) for pure PDMS and composites with 1wt% and 5 wt% ZnO inclusions on cooling. The value for 10 MHz frequency is obtained from previously described ultrasonic measurements. Extracted parameters are shown in Table 1. As we see the values obtained from the Vogel-Fulcher master curve of ln(f) dependence on T show increased glass transition temperature and decreased activation energy with adding ZnO nanoparticles to the PDMS in comparison with the pure PDMS. The change of the Ea and Tg is due to the presence of ZnO nanoparticles influenced the network formation [2].

Figure 5. Experimental and fitted values of frequency versus the dielectric loss maxima temperature according to the Vogel-Fulcher equation (Eq. 1) for pure PDMS and composites with 1 wt% and 5 wt% ZnO inclusions on cooling.

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J. Belovickis et al. Table 1 Vogel-Fulcher fit parameters of dielectric relaxation in ZnO/PDMS nanocomposite

Material Pure PDMS 1% ZnO/PDMS 5% ZnO/PDMS

f0 (THz)

Ea (eV)

Tref (K)

9.0 0.46 0.19

0.063 0.043 0.040

129.4 137.0 137.5


4. Conclusion ZnO/PDMS composite dielectric investigations show that the losses in composites are strongly dependent of ZnO inclusions concentration, i.e. a composite containing higher concentration of fillers has a lower dielectric loss peak. Dielectric spectroscopic measurements in ZnO/PDMS composites combined with ultrasonic measurements revealed the existence of dielectric hysteresis between cooling and heating process. Analysis of the maximum of the dielectric losses according to the Vogel-Fulcher law shows that the glass transition temperature increases and the activation energy decreases with increase in ZnO nanoparticle concentration in the ZnO/PDMS composites.

Funding This work was supported by the Lithuanian Research Council under project MIP-068/ 2012.

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